In wavelength division multiplexing (WDM) optical communication systems a single optical fiber may be used to carry multiple optical signals. The multiple optical signals are multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate channels. Each channel may be at an associated wavelength that is separated from adjacent channels by a defined channel-spacing, e.g. according to a channel plan established by the International Telecommunications Union (ITU). The range of wavelengths that may be transmitted on the system is known as the system bandwidth. Systems may utilize their system bandwidth to carry a desired number of channels with desired modulation format and bit rate.
One example of a prior art WDM transmission system 100 is illustrated in FIG. 1. The illustrated WDM system 100 includes first 102 and second 104 transceivers, an optical cable 106, and optical amplifiers 108-1 . . . 108-n. The optical cable 106 includes at least one pair of optical fibers 110e, 110w and a power conductor 112 for carrying electrical power to components coupled to the cable 106.
The system 100 serves to transmit optical signals TX1, TX2, TX3 from the first transceiver 102 in an “east” direction over fiber 110e to the second transceiver 104, where they are reproduced as received signals RX1, RX2, RX3, respectively. The system 100 also serves to transmit optical signals TX4, TX5, TX6 from the second transceiver 104 in a “west” direction over the fiber 110w to the first transceiver 102, where they are reproduced as received signals RX4, RX5, RX6, respectively.
Each of the amplifiers 108-1 . . . 108-n includes an erbium doped fiber amplifier (EDFA) 114-1 . . . 114-n, respectively, coupled to the “east” direction fiber 110e and an EDFA 116-1 . . . 116-n, respectively, coupled to the “west” direction fiber 110w for amplifying WDM signals on the fibers 110e and 110w. As is known, a rare-earth doped optical amplifier, such as an EDFA, operates by passing an optical signal through a doped fiber segment, and “pumping” the segment with light from another source such as a laser. The pump source excites rare-earth atoms, e.g. erbium atoms in the case of an EDFA, in the doped segment, which then serve to amplify the optical signal passing through the EDFA.
Within each amplifier 108-1 . . . 108-n the EDFAs 114-1 . . . 114-n and 116-1 . . . 116-n, respectively, are pumped by a common optical pump unit (OPU) 118-1 . . . 118-n to cause amplification of the WDM signals passing through the EDFAs 114-1 . . . 114-n and 116-1 . . . 116-n on the optical fibers 110e, 110w. Each of the OPUs 118-1 . . . 118-n includes a plurality of pumps. In general, the outputs of the pumps are combined and then split to provide two or more pump outputs for each OPU 118-1 . . . 118-n. The output power at each output of the OPUs 118-1 . . . 118-n is thus a combination of the pump power provided by each of the plurality of pumps in the OPU 118-1 . . . 118-n. Advantageously, if one of the pumps in the OPU 118-1 . . . 118-n fails, pump power from the other pump(s) with in the OPU 118-1 . . . 118-n is still provided at the outputs of the OPU 118-1 . . . 118-n for pumping the EDFAs 114-1 . . . 114-n and 116-1 . . . 116-n, respectively. Use of a common OPU 118-1 . . . 118-n for the EDFAs 114-1 . . . 114-n and 116-1 . . . 116-n, respectively, in each amplifier 108-1 . . . 108-n thus provides redundancy of pump power for pumping the EDFAs 114-1 . . . 114-n and 116-1 . . . 116-n within each amplifier 108-1 . . . 108n. 
One example of an OPU 118a is illustrated in FIG. 2. The illustrated OPU 118a includes three pumps 202, 204, 206, e.g. continuous wave lasers, a combiner 208 and a coupler 210. The outputs of first 202 and second 204 pumps are coupled as separate inputs to the combiner 208. The combiner 208 may, for example, be a polarization maintaining combiner. The combiner 208 combines the outputs of the first 202 and second 204 pumps to provide a combiner output. The combiner output is coupled as a first input the coupler 210, and the output of the pump 206 is coupled as a second input to the coupler 210. The coupler 210 may, for example, be a known 50/50 4-port coupler. The coupler 210 combines the combiner output and the output of the pump 206 and then splits the combined output onto two separate output paths 212, 214 as first and second outputs of the OPU 118a. Each of the pumps 202, 204, 206 is thus coupled to both the first and second outputs of the OPU 118a. This provides redundancy in the event of failure of one of the pumps 202, 204, 206, since if one or more of the pumps 202, 204, 206 fail, then each output of the OPU 118 includes pump power from the remaining pump(s) 202, 204, 206.
With reference again to FIG. 1, electrical power for driving the pumps within the OPUs 118-1 . . . 118-n is coupled to the OPU through the power conductor 112 in the cable 106. In the illustrated example, power feed equipment (PFE) 120 in the transceiver 102 supplies the electrical power to the power conductor 112. The system 100 may be described as a power-limited system since the maximum power Pmax that the PFE equipment 120 may deliver to the power conductor 112 is limited by length (determined by distance between the transceivers) and configuration (e.g., composition, diameter etc.) of the power conductor 112. Increasing the total power delivered to the power conductor 112 to more than Pmax jeopardizes the integrity of the power conductor 112 and/or the PFE 120. Since the system 100 is a power-limited system, the electrical power available for driving the pumps in the OPUs 118-1 . . . 118-n is limited, which limits the amount of total pump power that may be provided by the OPUs 118-1 . . . 118-n. 
The limited electrical power available in power-limited systems has created significant challenges to increasing transmission capacity. For example, multi-level modulation techniques and coherent receivers have been used to increase transmission rates and decrease channel spacing, thereby increasing the spectral efficiency (SE) of each channel in a WDM system. While use of multi-level modulation formats may increase spectral efficiency and transmission capacity, such formats may require increased signal-to-noise ratio (SNR). Operating with high SNR requires high optical channel output power and high amplifier pump power, especially for wide system bandwidths. In some configurations, delivering the required high power levels may not be technically and/or economically possible in a power-limited system.
Another approach to increasing spectral efficiency is to implement spatial division multiplexing (SDM). In an SDM system, a multi-dimensional fiber, e.g. a multi-core or multi-mode fiber, may be used, and the WDM signal may be separated onto each of the dimensions of the fiber. For example, instead of transmitting a WDM signal on a single core fiber, in an SDM system the signal may be separated and transmitted on each of the cores of a multi-core fiber or each of the modes of a multi-mode fiber.
Unfortunately, in long-haul SDM systems each of the dimensions, e.g. cores, of the transmission fiber must be amplified. In contrast to a non-SDM system wherein an WDM signal is carried on a single optical fiber and each amplifier amplifies the entire system bandwidth on the fiber, amplifying each of the dimensions in an SDM system requires a separate optical amplifier for each dimension. The optical pump power required to pump each of the dimensions of a multi-dimensional fiber may not be available in a power-limited system.